Research ArticleBasic Science Investigations

99mTc Radiotracers Based on Human GRP(18-27): Synthesis and Comparative Evaluation

Panteleimon J. Marsouvanidis, Theodosia Maina, Werner Sallegger, Eric P. Krenning, Marion de Jong and Berthold A. Nock
Journal of Nuclear Medicine October 2013, 54 (10) 1797-1803; DOI: https://doi.org/10.2967/jnumed.112.118695

Abstract

Gastrin-releasing peptide receptors (GRPRs) expressed on human tumors can serve as molecular targets for radiolabeled peptide analogs based on the frog tetradecapeptide bombesin (BBN). We have recently expanded this approach toward human GRP(18-27) sequences and introduced 99mTc-demomedin C, our first radiotracer based on GRP(18-27), showing favorable biologic characteristics during preclinical evaluation in rodents. We now present a series of 99mTc-demomedin C analogs, generated by single-Gly24 or double-Gly24/Met27 substitutions in the peptide chain, and compare their performance in GRPR-positive in vitro and in vivo models. Methods: The SARNC ([(N4)Gly18]GRP(18-27)) analogs (SARNC2 dAla24, SARNC3 dAla24/Nle27, SARNC4 dAla24/Leu27, SARNC5 βAla24, and SARNC6 Sar24) were synthesized on the solid support and purified by high-performance liquid chromatography (HPLC). Competition binding experiments against [125I-Tyr4]BBN were conducted in GRPR-positive PC-3 cell membranes. Internalization of 99mTc radioligands was compared in PC-3 cells at 37°C. Metabolic stability was studied by HPLC analysis of blood samples collected 5 min after injection of radiopeptides in mice. Biodistribution was performed by injecting a 99mTc-SARNC bolus (185 kBq [5 μCi], 100 μL, 10 pmol of peptide ± 40 nmol of Tyr4-BBN: in vivo GRPR blockade) in severe combined immune deficient mice bearing PC-3 xenografts. Results: SARNCs bound to GRPR with high affinity (range of 50% inhibitory concentration [IC50] values, 0.3 nM [SARNC5] to 9.3 nM [SARNC4]). 99mTc-SARNCs specifically internalized in PC-3 cells, with 99mTc-SARNC5 displaying the fastest internalization rate. 99mTc-SARNCs showed distinct degradation rates (17% [99mTc-SARNC3] to >50% [99mTc-SARNC4] remaining intact). All 99mTc-SARNCs efficiently and specifically localized in GRPR-positive PC-3 xenografts in mice (4.4 percentage injected dose per gram [%ID/g] [99mTc-SARNC4] to 12.0 %ID/g [99mTc-SARNC2] at 4 h after injection). 99mTc-SARNC6 displayed the highest tumor-to-nontumor ratios followed by 99mTc-SARNC2. Conclusion: This structure–activity relationship study has shown the impact of single-Gly24 or double-Gly24/Met27 substitutions in the 99mTc-SARNC1 motif on key biologic parameters, including GRPR affinity, internalization efficiency, and in vivo stability, which eventually determine the pharmacokinetic profile of resulting radiopeptides. By revealing improved analogs, this study has strengthened the applicability perspectives of radioligands based on human GRP sequences in the detection and therapy of GRPR-expressing tumors in humans.

Analogs of the frog tetradecapeptide bombesin (BBN) have been exploited as molecular vehicles to direct diagnostic and therapeutic radionuclides to human primary and metastatic cancer (1–3). This approach relies on the high-density expression of gastrin-releasing peptide receptors (GRPRs) in many frequently occurring human cancers, such as prostate cancer, mammary carcinoma, or small cell lung cancer, as opposed to their lower abundance or lack of expression in surrounding healthy tissue (4–9). The success of radiolabeled BBN analogs to target GRPR-expressing cancer lesions in animal models and in humans has been partly attributed to their ability to internalize rapidly and massively into cancer cells after receptor binding. Internalization enhances trapping of the radiolabel into malignant cells, a process translating into higher diagnostic sensitivity or superior therapeutic efficacy depending on the radionuclide used for labeling (10,11).

Following this rationale, we have previously reported on a series of tetraamine-functionalized 99mTc radiotracers based on the full-length BBN and its truncated BBN(7-14) C-terminal octapeptide fragment, 99mTc-demobesin 3-6. These analogs indeed showed high affinity for the human GRPR, rapid internalization into human prostate cancer PC-3 cells, and effective targeting of PC-3 tumors xenografted in immunosuppressed mice, whereas their excretion from the body of mice was found to be dependent on peptide length or hydrophilicity of spacers introduced between metal-chelate and peptide chain (11). Urged by the lack of studies on radioligands based on human homologs of amphibian BBN, we have recently expanded our research activities toward human peptide sequences, such as the 27-mer GRP and its C-terminal decapeptide fragment GRP(18-27), otherwise referred to as neuromedin C (3,12). Hence, we have recently introduced 99mTc-demomedin C, formed by coupling an acyclic tetraamine chelator to the primary amine of Gly18 of GRP(18-27) to allow for labeling with 99mTc (13). The new radioligand achieved high levels of specific uptake in PC-3 xenografts in severe combined immune deficient (SCID) mice while clearing more quickly from background tissues via the kidneys and into urine than the frog BBN–derived 99mTc-demobesin 3-6 (11,13). Thus, the excellent pharmacokinetic profile of 99mTc-demomedin C has established that human GRP sequences can be exploited at least as successfully as their frog homologs to direct radionuclides on GRPR-positive lesions in vivo, providing a new platform for further structural interventions.

For this purpose, we now present 5 new analogs of the 99mTc-demomedin C motif (99mTc-SARNC1) created by single-Gly24 or double-Gly24/Met27 substitutions to afford 99mTc-SARNC2 to 99mTc-SARNC6. Specifically, Gly24 substitution by dAla (SARNC2), βAla (SARNC5), or Sar (SARNC6) aimed toward higher metabolic stability of resulting radioligands without compromising GRPR affinity. Similar modifications on BBN-based motifs have led to good GRPR affinity (3,14), whereas a few BBN-based radioligands have shown a higher stability during incubation in mouse serum after replacement of the respective Gly11 in the frog tetradecapeptide chain (15). Two more analogs were further modified at position 27 with either Nle (SARNC3 dAla24/Nle27) or Leu (SARNC4 dAla24/Leu27), replacing the oxidation-susceptible Met in the native GRP(18-27) sequence. The effects of these modifications on key biologic parameters—such as GRPR affinity, internalization efficacy, metabolic stability, and pharmacokinetic profile—were studied in a head-to-head comparison using GRPR-expressing cell preparations and animal models and are reported herein. Results correlated with data reported for frog BBN–related radioligands, and conclusions on the applicability of new radiotracers in the scintigraphic detection of GRPR-positive lesions in patients are drawn.

MATERIALS AND METHODS

Synthesis of SARNCs

Synthesis of GRP(18-27) analogs was performed on an automated synthesizer (PSSM 8; Shimadzu) on a Tenta Gel S Ram resin (Rapp Polymere GmbH) as solid support (capacity, 0.25 mEq/g of resin). The 9-fluorenylmethoxycarbonyl (Fmoc) amino acid derivatives were supplied by Orpegen Pharma and were protected with Nα Fmoc and Nβ Fmoc in the case of βAla. Trt side chain–protected His and Asn were used. The elongation was performed by the coupling of a 10-fold excess of Fmoc amino acid derivatives in the presence of 1-hydroxybenzotriazole, N,N′-diisopropylcarbodiimide, and diisopropylamine in a mixture of N,N-dimethylformamide (DMF) and dichloromethane 90:10 v/v. After each coupling step, Fmoc deprotection was achieved with 30% piperidine in DMF. The assembled amino acid sequences were coupled with a 3-fold excess of the tetra-Boc–protected tetraamine chelator ((Boc-N)4-COOH, N,N′,N″,N′″-tetrakis-(tert-butoxycarbonyl)-6-(carboxy)-1,4,8,11-tetraazaundecane), (benzotriazol-1-yloxy)-tri-pyrrolidinophosphonium hexafluorophosphate, and diisopropylamine in DMF on the resin. Cleavage of the conjugates from the solid support was achieved by treating the fully protected peptide chains with a mixture of trifluoroacetic acid (TFA), 1,2-ethanedithiol, thioanisol, and H2O in a ratio 90:4:4:2 v/v/v/v at room temperature. After the resin was removed by filtration, the crude peptides were collected by precipitation with ice-cooled diethylether. Finally, the crude products were purified by reversed-phase high-performance liquid chromatography (RP-HPLC) on an AKZO Nobel Kromasil Semi/Prep C18 column (250 × 20 mm). Fractions containing the desired peptide were collected, and the solvent was removed by lyophilization. Analytic RP-HPLC data (from an Agilent system equipped with a Nucleosil-100 C18 column, 150 × 4 mm) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry results (MALDI-TOF MS, Kompact Kraton Axima Analytic; Shimadzu) are summarized in Table 1.

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TABLE 1

Analytic Data for SARNCs

99mTc Labeling and Quality Control of 99mTc-SARNCs

The lyophilized peptide analogs were dissolved in 50 mM acetic acid/EtOH 8/2 v/v to a final 1 mM concentration and stored at −20°C in 50-μL aliquots. Elution of a 99Mo/99mTc generator (Ultratechnekow; Tyco Healthcare) yielded 99mTcO4−, and labeling with 99mTc was conducted, as previously described (13). Radioanalytic HPLC was performed on a chromatograph coupled to a 996 photodiode array ultraviolet detector (Waters) and a Gabi γ-detector (Raytest RSM Analytische Instrumente GmbH). For analysis, a Waters RP8 XTerra (5 μm, 4.6 × 150 mm) cartridge column was eluted at a 1.0 mL/min flow rate with the following gradient: 0% B to 40% B in 20 min, where A = 0.1% aqueous TFA and B = MeCN. Under these conditions, 99mTcO4− elutes at 1.8 min and 99mTc-SARNCs with a retention time (tR) more than 13 min. For the detection of reduced hydrolyzed technetium (99mTcO2 × H2O), instant thin-layer chromatography (ITLC) was conducted on ITLC silica gel strips (Gelman Science), as previously described (11,13).

In Vitro Assays

Human androgen-independent prostate adenocarcinoma PC-3 cells spontaneously expressing the GRPR (16) (LGC Promochem) were cultured as previously reported (11,13) and were used in biologic assays. Competition binding experiments were conducted in PC-3 cell membranes as previously described (13). Tyr4-BBN (PSL GmbH) and 125I (MDS Nordion, SA) were used for preparing [125I-Tyr4]BBN. The latter served as a radioligand (∼40,000 cpm per assay tube, at a 50 pM concentration) and GRP(18-27) (PeptaNova) as a reference compound; incubation at 22°C was applied for 45 min in an Incubator-Orbital Shaker (MPM Instruments SrI). Samples were measured for radioactivity in an automatic well-type γ-counter ([NaI(Tl)] crystal; Auto-γ-5000 series instrument [Canberra Packard]). The 50% inhibitory concentration (IC50) values were extracted from at least 3 independent experiments performed in triplicate using nonlinear regression according to a one-site model applying the PRISM 2 program (GraphPad Software).

For internalization, confluent PC-3 cells were seeded in 6-well plates (∼1.0 × 106 cells per well) 24 h before the experiment was conducted. Approximately 300,000 cpm of test 99mTc-SARNC (corresponding to 200 fmol of total peptide in 150 μL of 0.5% bovine serum albumin/phosphate-buffered saline) was added alone (total) or in the presence of 1 μM Tyr4-BBN (nonspecific), and the experiment was performed following a published protocol (11). Results were calculated as percentage internalized per total added activity per million cells for each time point using Microsoft Excel and represent the average of at least 2 experiments performed in triplicate.

Metabolism of 99mTc-SARNCs

99mTc-SARNC was injected as a 100-μL bolus (55.5–111 MBq [1.5–3.0 mCi], 3 nmol of total peptide) in the tail vein of male Swiss albino mice (30 ± 5 g, NCSR “Demokritos” Animal House Facility). Mice were anesthetized with ether, and blood (0.5–1 mL) was collected from the heart at exactly 5 min after injection. Blood samples were immediately transferred in prechilled polypropylene tubes containing ethylenediaminetetraacetic acid and placed on ice. Samples were prepared for analysis by HPLC as previously described (17). The Waters Symmetry Shield RP18 (5 μm, 3.9 × 20 mm) column was eluted at a flow rate of 1.0 mL/min with the following gradient: 100% A to 90% A in 10 min and from 90% A to 60% for the next 60 min; for 99mTc-SARNC3 the gradient progressed from 100% A to 40% A within 60 min (A = 0.1% aqueous TFA [v/v] and B = MeCN). ITLC was performed in parallel using acetone as the eluent to detect traces of 99mTcO4− release (99mTcO4− Rf = 0.9).

Biodistribution of 99mTc-SARNCs in PC-3 Xenograft–Bearing Mice

An approximately 150-μL bolus containing a suspension of approximately 1.5 × 107 freshly harvested human PC-3 cells in saline was subcutaneously injected in the flanks of female SCID mice (weight ± SD, 15 ± 3 g; age at the day of arrival, 6 wk; NCSR “Demokritos” Animal House Facility). The animals were kept under aseptic conditions and 2–3 wk later developed well-palpable tumors at the inoculation site (80–150 mg). On the day of the experiment, the selected 99mTc-SARNC was injected in the tail vein of mice as a 100-μL bolus (185 kBq [5 μCi], 10 pmol of total peptide; in saline/EtOH 9/1 v/v), and biodistribution for the 1-, 4-, and 24-h postinjection time intervals was conducted as previously described (11,13). For in vivo GRPR blockade, a separate 4-h animal group additionally received excess Tyr4-BBN (40 nmol). Biodistribution data were calculated as percentage injected dose per gram of tissue (%ID/g) using the Microsoft Excel program and with the aid of suitable standards of the injected dose.

Statistical analysis using the unpaired 2-tailed Student t test was performed to compare values between control and the in vivo GRPR-blockade animal groups at 4 h after injection; P values of less than 0.005 were considered statistically highly significant.

All animal experiments were performed in compliance with European and national regulations and after approval of protocols by national authorities.

RESULTS

SARNC Ligands and 99mTc-SARNC Radioligands

All 6 GRP(18-27) sequences were assembled on the solid support following typical Fmoc-protection methodology, and the Boc-protected N4-COOH chelator was attached to their N terminus in the last cycle. After release from the resin and removal of lateral protecting groups with TFA treatment, SARNCs (Fig. 1) were isolated by chromatographic methods in 95% purity or greater, as shown by analytic HPLC; MALDI-TOF spectra of the products were consistent with the expected formulae (Table 1).

FIGURE 1.
FIGURE 1.

Chemical structures of SARNCs: SARNC1 (demomedin C motif)—Xaa24/Xaa27 = Gly24/Met27; SARNC2—dAla24/Met27; SARNC3—dAla24/Nle27; SARNC4—dAla24/Leu27; SARNC5—βAla24/Met27; and SARNC6—Sar24/Met27.

99mTc-SARNC radioligands were obtained after 30-min incubation of respective SARNCs with 99mTcO4− and SnCl2 in the presence of citrate anions in alkaline aqueous medium, whereby complete incorporation of 99mTc by the tetraamine framework was accomplished. Labeling yields greater than 98% were verified by combined ITLC and RP-HPLC methods in a specific activity of 18.5–37 MBq (0.5–1 mCi)/nmol of SARNC.

In Vitro Studies

The binding affinities of SARNCs for the human GRPR were determined in PC-3 cell membranes. As shown in Figure 2, SARNCs were able to displace [125I-Tyr4]BBN from GRPR sites on PC-3 membranes in a monophasic and dose-dependent manner. The calculated IC50 values were in the range of 0.28 ± 0.02 nM (SARNC5) to 9.29 ± 3.55 nM (SARNC4), with the IC50 for native GRP(18-27) determined at 1.66 ± 0.2 nM. Single-Gly24 substitutions had a minor impact on receptor affinity as compared with the SARNC1 motif (IC50 = 0.73 ± 0.42 nM, curve not included in the diagram), revealing a positive effect in the case of βAla24 replacement (SARNC5). Additional Met27 substitutions led to less affine conjugates, especially in the case of the dAla24/Leu27 combination (SARNC4).

FIGURE 2.
FIGURE 2.

Displacement of [125I-Tyr4]BBN from GRPR sites in PC-3 cell membranes by increasing concentrations of test peptide: ◆= SARNC2 (IC50 = 2.03 ± 1.06 nM); ■ = SARNC3 (IC50 = 2.96 ± 1.33 nM); • = SARNC4 (IC50 = 9.29 ± 3.55 nM); ▲ = SARNC5 (IC50 = 0.28 ± 0.02 nM); and ▼ = SARNC6 (IC50 = 1.78 ± 0.32 nM). Control: * = GRP(18-27) (IC50 = 1.66 ± 0.12 nM). Results represent average IC50 values ± SD of 3 independent experiments performed in triplicate.

Internalization rates of 99mTc-SARNCs in PC-3 cells are compared in Figure 3. 99mTc-SARNC5 (βAla24 analog) showed the highest internalization efficiency, consistent with its superior receptor affinity. Slower internalization rates were exhibited by all other 99mTc-SARNCs in line with their respective affinities for the GRPR. As a result, the bis-Gly24/Met27–substituted analogs 99mTc-SARNC3 and 99mTc-SARNC4 displayed the poorest internalization efficacy among this series of analogs, but still approximately 80% of cell-associated radioactivity was internalized, consistent with an agonist profile. In all cases, internalization dropped below 1% in the presence of 1 μM Tyr4-BBN, indicating a GRPR-mediated process.

FIGURE 3.
FIGURE 3.

Comparative internalization of 99mTc-SARNCs in PC-3 cells at 37°C as function of time: * = 99mTc-SARNC1; ♦ = 99mTc-SARNC2; ■ = 99mTc-SARNC3; • = 99mTc-SARNC4; ▲ = 99mTc-SARNC5; and ▼ = 99mTc-SARNC6. Results represent percentage of specific internalized of total added radioactivity per million cells, and percentage of nonspecific internalization (<1% in all cases in presence of 1 μM Tyr4-BBN) is indicated by empty bullets; assays have been performed twice in triplicate.

Stability and Biodistribution of 99mTc-SARNCs in PC-3 Xenograft–Bearing Mice

The radiopeptide metabolism after entry into the bloodstream of mice was studied by RP-HPLC analysis of blood samples collected 5 min after injection. Representative radiochromatograms shown in Figure 4 reveal distinct radiometabolite patterns and degradation rates for individual 99mTc-SARNCs. 99mTc-SARNC1 shows a much faster in vivo degradation (30% intact at 5 min after injection) than its previously reported in vitro breakdown in mouse serum at 37°C (65% intact at 30 min) (13). Radioligand integrity was moderately prolonged after Gly24 substitution by dAla (40%), βAla (32%), and Sar (42%) as compared with unmodified 99mTc-SARNC1. On the other hand, double Gly24/Met27 substitution produced the most stable (>50%) 99mTc-SARNC4 (dAla24/Leu27 combination) and the least stable radioligand (17%) 99mTc-SARNC3 (dAla24/Nle27 combination) within this group of analogs.

FIGURE 4.
FIGURE 4.

Radiochromatograms of blood samples collected 5 min after injection in mice for 99mTc-SARNC1 (30% intact, tR = 23.6 min) (A), 99mTc-SARNC2 (40% intact, tR = 25.0 min) (B), 99mTc-SARNC3 (17% intact, tR = 23.0 min) (C), 99mTc-SARNC4 (51% intact, tR = 28.5 min) (D), 99mTc-SARNC5 (32% intact, tR = 22.9 min) (E), and 99mTc-SARNC6 (42% intact, tR = 24.9 min) (F). Coinjection of samples with aliquots of labeling reaction solutions in HPLC revealed the position of parent radiopeptides (indicated by green arrow); HPLC conditions and gradient systems applied are given in text.

Cumulative biodistribution data of 99mTc-SARNCs in SCID mice bearing human GRPR-positive PC-3 xenografts are summarized in Table 2 (99mTc-SARNC1 included for comparison purposes (13) and 99mTc-SARNC2), Table 3 (99mTc-SARNC3 and 99mTc-SARNC4), and Table 4 (99mTc-SARNC5 and 99mTc-SARNC6). Results represent average %ID/g values with SD at 1, 4, and 24 h after injection. All analogs showed a fast blood clearance. In all cases, radioactivity washed out from the body of mice via the kidneys rapidly into urine, with excretion via the hepatobiliary pathway playing a minor role.

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TABLE 2

Biodistribution Data of 99mTc-SARNC1 and 99mTc-SARNC2 in PC-3 Xenograft–Bearing SCID Mice

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TABLE 3

Biodistribution Data of 99mTc-SARNC3 and 99mTc-SARNC4 in PC-3 Xenograft–Bearing SCID Mice

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TABLE 4

Biodistribution Data of 99mTc-SARNC5 and 99mTc-SARNC6 in PC-3 Xenograft–Bearing SCID Mice

All analogs were able to specifically target the GRPR-positive xenografts and mouse pancreas, as demonstrated by the significant reduction in the corresponding uptake values observed in the animals treated with excess Tyr4-BBN (in vivo GRPR blockade). The highest tumor uptake was exhibited by 99mTc-SARNC2 (12.05 ± 1.22 %ID/g at 4 h after injection), followed by 99mTc-SARNC5 (9.57 ± 0.33 %ID/g at 4 h after injection) and 99mTc-SARNC6 (9.22 ± 1.40 %ID/g at 4 h after injection), which are the analogs that had undergone single substitutions at position 24 by dAla, βAla, or Sar, respectively. On the other hand, double-Gly24/Met27–substituted 99mTc-SARNC3 and 99mTc-SARNC4 displayed much lower tumor uptake at the same time intervals (≈4.5 %ID/g at 4 h after injection). A similar trend was observed in pancreatic uptake, with less accumulation in the mouse pancreas of the twice Gly24/Met27-substituted members than in their Gly24-substituted counterparts. It is interesting to observe the massive pancreatic uptake of the βAla24 analog versus the low pancreatic uptake of the Sar24 analog (93.9 vs. 14.5 %ID/g at 4 h after injection, respectively) as opposed to their similar tumor uptake, revealing a most favorable tumor-to-background profile for 99mTc-SARNC6.

DISCUSSION

In recent years, amphibian BBN–based radioligands have been studied at length as molecular tools to diagnose and treat GRPR-positive cancer (e.g., prostate, breast, and lung) in humans (1,2,9,12,15). In principle, this approach can be successful in targeting both primary and metastatic disease not only when the high density of GRPR expression in malignant lesions is maintained but also when the applied radioligands fulfill important prerequisites, such as availability in high specific activity, good receptor affinity and internalization ability, stability in the biologic milieu, and favorable pharmacokinetics to achieve high tumor-to-background ratios. Although native homologs of BBN do exist in humans, to date research efforts have been exclusively focused on frog BBN and its derivatives. BBN shows high binding affinity for 2 of the 3 human bombesin receptor (BBR) subtypes, the neuromedin B receptor (NMBR, BB1R) and the GRPR (BB2R), but no affinity for the orphan BB3-receptor subtype (12,14).

It has been shown that 2 mammalian homologs of BBN, the 27-mer GRP and its C-terminal decapeptide fragment GRP(18-27) (otherwise known as neuromedin C), demonstrate high-affinity binding preferably to the human GRPR. Consequently, human GRP sequences can be exploited as molecular vectors, supplementary to the frog-derived motifs exclusively considered thus far, to tag diagnostic and therapeutic radionuclides on GRPR-expressing cancer. Following this rationale, we have recently introduced 99mTc-demomedin C, the first radioligand based on a human sequence, generated by coupling an acyclic tetraamine chelator to Gly18 of GRP(18-27) followed by labeling with 99mTc. 99mTc-demomedin C displayed an excellent pharmacokinetic profile in mice bearing human PC-3 xenografts, characterized by high and specific uptake in the GRPR-positive xenografts and by a fast body clearance via the kidneys and the urinary tract (13). This in vivo profile was found to be superior to similarly modified 99mTc-demobesin 3-6, which are based on full tetradecapeptide BBN and its C-terminal nonapeptide fragment BBN(6-14) (11), especially with regards to longer tumor retention and faster body clearance.

Motivated by these promising results, we have now attempted additional structural modifications on the 99mTc-demomedin C motif, in an effort to further improve the biologic profile of resulting radioligands. Modifications comprised substitutions of Gly24 alone (dAla24, βAla24, or Sar24) or in combination with Met27 (dAla24/Nle27 or dAla24/Leu27) to afford SARNC2 to SARNC6, as depicted in Figure 1. The above effects on GRPR affinity, internalization, in vivo stability, and biodistribution of resulting compounds in PC-3 tumor–bearing mice compared under the same experimental settings are discussed herein.

Thus, single-Gly24 replacements by dAla or Sar in the demomedin C (SARNC1) motif slightly deteriorated affinity for the GRPR, whereas βAla substitution turned out to be more advantageous in terms of GRPR affinity. Further Met27 replacement leading to either dAla24/Nle27 or dAla24/Leu27 analogs deteriorated GRPR affinity, with SARNC4 displaying the weakest binding affinity to the human GRPR (Fig. 2). These findings are in line with analogous effects on binding affinity for the human GRPR reported for similar modifications of frog BBN and BBN(6-14) sequences (3,14,15,18). Internalization efficacy of 99mTc radioligands followed the same trend, with 99mTc-SARNC5 (βAla24 replacement) displaying clearly superior internalization capacity at all time points, whereas 99mTc-SARNC4 (βAla24/Leu27 combination) internalized poorly in PC-3 cells at 37°C (Fig. 3).

Metabolic stability of 99mTc-SARNCs was studied by analysis of blood samples collected 5 min after injection. The degradation rate of unmodified 99mTc-SARNC1 in vivo was found to be much faster than during the in vitro incubation in mouse plasma reported previously, but a comparable pattern of radiometabolites was established. Similar observations between in vitro and in vivo degradation rates have also been reported for BBN-based radioligands (e.g., 177Lu-DO3A-CH2CO-G-4-aminobenzoyl-Q-W-A-V-G-H-L-M-NH2) (19,20). Radiopeptides that had undergone single-Gly24 or double-Gly24/Met27 substitutions showed improvement of stability, except for 99mTc-SARNC3 (βAla24/Nle27 combination), which degraded much more rapidly after entry into the mouse bloodstream, even more so than unmodified 99mTc-SARNC1. Although neutral endopeptidase 24.11 has been reported to cleave the Trp-Ala and His-Leu bonds in the conserved C-terminal heptapeptide chain of native BBN and GRP peptides (20,21), other enzymes may be implicated in their degradation as well (18,20). Radiometabolite patterns from 99mTc-SARNCs differ from those derived from similarly modified but BBN-based 99mTc-demobesin 3-6 (11). It is apparent that more thorough studies are required to elucidate the degradation pathways of BBN-/GRP-based radioligands, instrumental for the rational design of truly in vivo robust analogs.

The biodistribution profile of 99mTc-SARNCs in mice bearing PC-3 xenografts, especially the uptake in the experimental tumors, seems to be a combined result of the above-described biologic features, namely of in vitro receptor affinity, internalization efficacy, and metabolic stability. Indeed, uptake in the human GRPR-positive xenografts among the mono-Gly24–substituted analogs is highest for 99mTc-SARNC2, exhibiting improved stability (40% intact) and good receptor affinity (IC50 ≈ 2 nM). The uptake of 99mTc-SARNC5 (IC50 ≈ 0.3 nM) and 99mTc-SARNC6 (>40% intact) is a compromise between affinity and stability, eventually resulting in comparable tumor values at all time intervals. Likewise, the doubly substituted members showing either poor stability (99mTc-SARNC3, 17% remaining intact at 5 min after injection) or low affinity (99mTc-SARNC4, IC50 > 9 nM) end up with similarly poor uptake in the xenografted tumors.

Seen together with background clearance rate, these results reveal a superior profile for 99mTc-SARNC6, especially as far as kidney, intestine, and pancreatic values are concerned (Tables 2 and 3). The opposite holds true for 99mTc-SARNC5, which exhibits the highest uptake in all the above physiologic tissues among the 99mTc-SARNC members (13). This finding is intriguing and may reflect differences in BBR-subtype selectivities between the Sar24- and the βAla24-substituted analog. Analogous substitution of Gly by βAla in GRP (at position 24) or in BBN (at position 11) sequences have led to a broader BBR-affinity profile (3,14). The universal radiotracer 125I-[dTyr6,βAla11,Phe13,Nle14]BBN(6-14) is likewise substituted in the corresponding position 11 of the BBN-tetradecapeptide motif by βAla (22). An alternative explanation for this difference may be assigned to interspecies homology and tissue expression differences reported between the mouse and the human GRPR (14,15,23,24). These issues are currently addressed in detail by ongoing studies, and results will be instrumental in the design of the next generation of analogs based on either human or frog sequences.

CONCLUSION

The lack of studies on radioligands based on human GRP motifs for diagnosis and treatment of GRPR-positive cancer in humans has prompted us to recently introduce 99mTc-demomedin C, the first GRP(18-27)-based radiotracer. We herein present a small library of GRP(18-27) analogs, likewise coupled to acyclic tetraamines to allow for 99mTc labeling, that have undergone single (Gly24) or double (Gly24/Met27) substitutions (SARNC1–SARNC6) and compare their performance in GRPR-positive in vitro and in vivo models. This study has shown that analogs of highest receptor affinity (SARNC5-βAla24) or metabolic stability (SARNC4-dAla24/Leu27) alone did not lead to the most favorable in vivo profile. A superior in vivo profile in terms of PC-3 tumor uptake and fast body clearance in mice was accomplished by 99mTc-SARNC6 (Sar24 analog), which represents the best combination of GRPR affinity and metabolic stability within the group.

DISCLOSURE

The costs of publication of this article were defrayed in part by the payment of page charges. Therefore, and solely to indicate this fact, this article is hereby marked “advertisement” in accordance with 18 USC section 1734. No potential conflict of interest relevant to this article was reported.

Footnotes

  • Published online Sep. 5, 2013.

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  • Received for publication December 18, 2012.
  • Accepted for publication April 12, 2013.
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